In directed countermeasure systems, an incoming missile is detected by an IR focal plane array in a target acquisition mode and when detected is tracked in a tracking mode. In the target acquisition mode, it is important that the optics utilized in detecting the launch of, for instance, a shoulder-fired missile, have a relatively wide field of view so that incoming missiles will not escape the countermeasure system by being outside its field of view. For such a target acquisition, the lens assembly utilized has oftentimes been likened to a fisheye lens which in essence covers a wide field of view and is therefore capable of detecting targets over a wide area.
Early countermeasure systems included a simple fixed field of view optical system in order to acquire incoming missiles which were either shoulder launched or launched from vehicles. As will be appreciated, these early countermeasure systems worked to determine whether or not a missile had been fired against an aircraft, or other target. The aircraft was provided with sensors to detect the plume of the exhaust gases from the missile to warn those in the aircraft that a missile had been launched at the aircraft.
Early on, various systems were utilized to separate false targets from a real target and then to direct a jam head to slew over to the position of the detected target, whereupon jamming radiation was emitted from the jam head.
The problem with the fixed field of view was that in order to be able to find a threat one needed a wild field of view; but in order to track the threat one needed a relatively small field of view. If one were limited to a narrow field of view for the resolution necessary for tracking, the system might not find incoming missiles off-axis by more than a couple of degrees. The result is that one had to scan the narrow field-of-view optics to be able to detect an incoming threat. This search mode, however, was time consuming, leading to the present two field-of-view system described below.
Moreover, once the target had been detected, one had to position the jam head to put the detected target directly at the center of the optics of the jam head and then perform very precision tracking in order to be able to accurately detect the position in space of the incoming missile so that countermeasure radiation could be projected directly into the seeker or reticule of the incoming missile.
The problem with fixed field-of-view optics had been the tradeoff between the field of view being too small to acquire a target versus being too large to be able to accurately determine its position in space.
In order to solve the problem of field of view for the acquisition and tracking phases, in the past a technique was developed which involved a two field-of-view system.
The two field-of-view system was selected over a zoomed telephoto lens both because of the size of the telephoto lens and alignment problems when zooming. It will be appreciated that in the gimbaled head of the tracker, telephoto zoom-type optics require too much space, especially, for instance, for a telephoto lens to go from a wide field of view to a narrow field of view, sometimes involving a 5:1 ratio.
In short, it was impractical to provide a pod of sufficient dimensions to house such a telephoto lens and for this reason, current two field-of-view systems are now prevalent.
In these systems one has a fixed objective lens system which provides for the telephoto magnification utilized in the tracking phase in which the viewing angle is on the order of one to two degrees. In the search mode, a movable assembly is rotated onto the optical center line of the fixed objective lens. The purpose of interposing the assembly between the objective lens and the IR focal plane detector is to provide a combined focal length which is quite short, resulting in a wide field of view suitable for the search or acquisition mode. In one embodiment, during the search or acquisition phase, the effective focal length of the system was short to provide a wide field of view. In the tracking phase, the assembly was rotated out of the way of the objective lens system to increase the focal length for increasing magnification to assist tracking. In so doing, the effective focal length is several times greater. The result is magnification and better resolution, but at the cost of a narrow field of view unsuitable for searching.
In operation, the moveable assembly was rotated onto the center line of the fixed objective lens. The entire optical system was then driven by gimbals to center a target in the field of view. When centered in the field of view, the movable assembly was rotated away from the optical center line of the objective lens to switch to a higher magnification, with the higher magnification achieving sub-milliradian tracking accuracies.
Such a two field-of-view system was more than adequate to acquire the incoming missile targets in a search or acquisition phase and to later track the acquired targets to a high precision in the tracking phase.
The problem with such two-field-of-view systems is that the switching from the search phase to the tracking phase and back again takes a relatively long period of time, long relative to the time of flight of the attacking missile. Switching delays could regularly exceed a second. It will be noted that, for a shoulder-fired missile, oftentimes there is only a period of less than three seconds in which to identify that a missile has been fired, to locate it, to center the missile on the optical axis of the countermeasure detection system, to track the incoming missile with a high degree of precision and then to jam it with radiation projected out along the tracker's optical axis.
Not only were solenoids or motors used to drive the movable assembly into and out of position along the optical axis of the objective lens, alignment problems could occur during this electromechanical actuation process. Thus, for instance, if the optics were to initially aligned to within tight tolerances, oftentimes the movable assembly threw the entire system out of alignment.
More importantly, should the solenoid or mechanical actuation system fail at any time within the critical engagement period, one could not track the threat and the countermeasure system could fail to countermeasure the incoming missile.
There was therefore a requirement for providing a multiple focal length or multiple field-of-view system without moving parts. It was thought that making the multiple field-of-view system a fixed optical system would eliminate the delays associated with switching. It would also eliminate alignment errors as well as the effects of mechanical malfunction so as to provide a robust search and tracking system within an exceptionally small package.
In short, there was a requirement to have a fixed optical system to provide a wide field of view for acquisition and a narrow field of view for tracking.
Moreover, missiles are usually fired in pairs and the jammer must deal with them one at a time. One therefore needs a system that has a wide, continuous field of view so that it can both track and jam one missile and identify/characterize a second missile for subsequent jamming. This cannot be done with field switching systems that either acquire or track but do not do both simultaneously.